Inorg. Chem. 1997, 36, 567-570
567
Synthesis, Structure, and Magnetic Properties of New Ru26+ Compounds F. A. Cotton* and A. Yokochi Department of Chemistry and Laboratory for Molecular Science and Bonding Texas A&M University, College Station, Texas 77843 ReceiVed May 30, 1996X
Three compounds containing the Ru26+ core have been prepared by clean and convenient synthetic routes: [Ru2(ap)4(OH2)][CF3SO3]2, [Ru2(ap)4F][PF6], and [Ru2(ap)4Cl][FeCl4] (ap ) anilinopyridinate). The third one has been structurally characterized from a crystal of [Ru2(ap)4Cl][FeCl4]‚2.5CH2Cl2 and contains the Ru26+ core in a coordination environment identical to that of an Ru25+ compound. The compound crystallizes in the triclinic space group P1h, with a ) 9.660(2) Å, b ) 16.237(3) Å, c ) 17.724(4) Å, R ) 86.34(3)°, β ) 77.19(3)°, γ ) 78.34(3)°, and Z ) 2. The Ru-Ru distance is 2.301(1) Å, and the magnetic moment indicates the presence of two unpaired electrons. A σ2π4δ2π*2 electron configuration is inferred.
Introduction While compounds of Ru2n+ (n ) 4, 5) are easily prepared and have been extensively studied,1 their Ru26+ counterparts have proved elusive, with only five known in the current literature. The first bona fide series of Ru26+ compounds, containing the species [Ru2(SO4)4(H2O)2]2-, was reported in 1986 by Zhilyaev and co-workers,2 and was shortly thereafter confirmed by this laboratory.3 Recently, two new classes of compounds have been reported by Bear and co-workers.4 One, of general formula Ru2(LL)4(Lax)2, where (LL) is either diphenylformamidine or pentafluoroanilinopyridine and Lax is CCPhor CN-, and a second class with its only representative Ru2(hpp)4Cl2, where hpp is the 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidinate anion. The three classes of compounds have different magnetic properties: the first type has four unpaired electrons, those of the second type are diamagnetic, and the last one has two unpaired electrons. For some time we have been continuing to investigate the preparation of Ru26+ compounds and have synthesised three compounds of the third class. One of them has been studied by X-ray crystallography and contains the Ru26+ core in a coordination sphere similar to that of an Ru25+ homologue, Ru2(ap)4Cl (ap ) anilinopyridinate).5 A discussion of the structural changes caused by the oxidation will be provided. Experimental Section All the work described here was carried out under an atmosphere of dry N2 unless otherwise indicated. All solvents used were dried and deoxygenated prior to use and freshly distilled from an appropriate drying agent under an atmosphere of dry N2. Ru2(ap)4Cl was prepared according to published methods.5 Ag(CF3SO3) was purchased from X Abstract published in AdVance ACS Abstracts, January 15, 1997. (1) Cotton; F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms, 2nd ed.; Oxford University Press; Oxford, England, 1993. (2) (a) Zhilyaev, A. N.; Fomina, T. A.; Kuzmenko, I. V.; Rotov, A. V.; Baranovskii, I. B. Russ. J. Inorg. Chem. (Engl. Transl.) 1989, 34, 532. (b) Kuzmenko, I. V.; Zhilyaev, A. N.; Fomina, T. A.; Porai-Koshits, M. A.; Baranovskii, I. B. Russ. J. Inorg. Chem. (Engl. Transl.) 1989, 34, 1457. (3) Cotton, F. A.; Datta, T.; Labella, L.; Shang, M. Inorg. Chim. Acta 1993, 203, 55. (4) (a) Bear, J. L.; Han, B. .C.; Huang, S. R. J. Am. Chem. Soc. 1993, 115, 1175. (b) Li, Y. L.; Han, B. C.; Bear, J. L.; Kadish, K. M. Inorg. Chem. 1993, 32, 4175. (c) Bear, J. L.; Li, Y.; Han, B.; Kadish, K. M. Inorg. Chem. 1996, 35, 1395. (5) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1985, 24, 172.
S0020-1669(96)00637-4 CCC: $14.00
Aldrich Chemical Co. and Ag(PF6) and [FeCp2][FeCl4] were purchased from AESAR/Johnson Matthey and used as received. Elemental analyses were performed by Canadian Microanalytical Co. Infrared spectra were recorded on a Perkin Elmer FT-IR Model 1600 PC. Electronic absorption spectra were recorded on a Cary 17 spectrophotometer. Single-crystal X-ray diffraction data were collected on an Enraf Nonius FAST system. Mass spectrometry was carried out on a VG Analytical 70S high-resolution, double-focusing, sectored (EB) mass spectrometer. Bulk magnetic susceptibility was measured using a Johnson Matthey MSB-1 balance. In all cases the solids employed in the magnetic measurements and elemental analysis were dried under vacuum for several hours prior to use. [Ru2(ap4)(OH2)][O3SCF3]2. In a Schlenk flask equipped with a magnetic stirring bar were placed 0.10 g Ru2(ap)4Cl (0.11 mmol) and 0.06 g Ag(O3SCF3) (0.23 mmol). The flask was sealed with a rubber septum and all air excluded by repeatedly evacuating and refilling with dry N2. CH2Cl2 (20 mL) was then syringed into the flask through the rubber septum. A deep green solution formed initially which changed to dark red overnight. The solution was stirred for 12 h and then filtered through a fine glass frit to leave behind a dark gray solidspossibly a mixture of excess Ag(O3SCF3), AgCl, and metallic silver. A dark red solid was obtained by removing the solvent at reduced pressure. Yield: 0.115 g (96%). Crystals of this compound were obtained by layering a CH2Cl2 solution with hexanes, but due to extensive disorder, observed as disorder streaks in axial photographs, the structure has not been refined satisfactorily. Anal. Calcd for Ru2N8C46H38O7S2F6: C, 46.23; H, 3.20; N, 9.38; O, 9.37. Found: C, 46.02; H, 3.20; N, 9.02; O, 10.53. IR (KBr, cm-1): 3505 w, 2924 m, 1597 s, 1462 s, 1429 s, 1341 m, 1287 s, 1256 s, 1210 s, 1158 s, 1031 s, 924 m, 870 s, 802 w, 763 s, 735 w, 698 s, 636 s, 573 w, 517 w, 498 w. Vis-near-IR (CH2Cl2, nm): 395, 490, 940 (broad). [Ru2(ap4)F][PF6]. In a Schlenk flask equipped with a magnetic stirring bar were placed 0.10 g Ru2(ap)4Cl (0.11 mmol) and 0.07 g Ag(PF6) (0.28 mmol). The flask was sealed with a rubber septum and all air excluded by repeatedly evacuating and refilling with dry N2. CH2Cl2 (20 mL) was then syringed into the flask through the rubber septum. A deep green solution formed initially which rapidly changed to dark red. The solution was stirred for 12 h and then filtered through a fine glass frit to leave behind a dark gray solid. To the resulting solution was then added 5 mL of freshly distilled acetonitrile and the resulting solution layered with 10 mL of hexanes to afford small clusters of thin plate-shaped crystals. Yield: 0.094 g (90%). The compound was structurally characterized,6 but due to extensive disorder of the PF6- counterion, the structure has not been refined to a satisfactory conclusion at this moment, and may be reported elsewhere. Anal. Calcd for Ru2N8C44H36PF7: C, 50.67; H, 3.48; N, 10.74. Found: C, 49.57; H, 3.78; N, 10.08. IR (KBr, cm-1): 3058 w, 2924 m, 1598 s, 1463 s, 1429 s, 1339 m, 1287 m, 1259 w, 1209 s, 1161 w, 1113 w, 1072 w,
© 1997 American Chemical Society
568 Inorganic Chemistry, Vol. 36, No. 4, 1997
Cotton and Yokochi
Table 1. Crystal Data and Structure Refinement for [Ru2(ap)4Cl][FeCl4]‚2.5CH2Cl2 empirical formula: C46.50H41Cl10FeN8Ru2 fw ) 1324.41 space group: P1h unit cell dimens: a ) 9.660(2) Å, b ) 16.237(3) Å, c ) 17.724(9) Å, R ) 86.34(3)°, β ) 77.19(3)°, γ ) 78.34(3)° V ) 2654.4(9) Å3 Z)2 a
d(calcd) ) 1.657 g/cm3 abs coeff 1.376 mm-1 wavelength ) 0.71073 Å temp ) 213(2) K refinement method: full-matrix least-squares on F2 final R indices [I>2σ(I)]: R1a ) 0.073; wR2b ) 0.161 R indices (all data): R1a ) 0.103; wR2b ) 0.179
R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) |∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]0.5.
1024 w, 922 w, 868 s, 839 s, 766 m, 738 w, 697 m, 557 m, 501 w. Vis-near-IR (CH2Cl2, nm): 395, 490, 940 (broad). [Ru2(ap4)Cl][FeCl4]. In a Schlenk flask equipped with a magnetic stirring bar were placed 0.10 g Ru2(ap)4Cl (0.11 mmol) and 0.05 g [FeCp2][FeCl4] (0.13 mmol). The flask was sealed with a rubber septum and all air excluded by repeatedly evacuating and refilling with dry N2. CH2Cl2 (20 mL) was then syringed into the flask through the rubber septum. A deep green solution formed initially, but the color immediately changed to dark red. The solution was stirred for 1 h and the solvent removed by evaporation at reduced pressure. The resulting solid was washed with several 5 mL portions of freshly distilled hexanes until no yellow color attributable to dissolved ferrocene could be detected by eye. Yield: 0.107 g (96%). The solid was then redissolved in CH2Cl2 and layered with 30 mL of hexanes. Over the course of several days, very small, thin crystals formed. Anal. Calcd for Ru2N8C44H36FeCl5‚CH2Cl2: C, 45.15; H, 3.20; N, 9.36; Cl, 20.73. Found: C, 46.14; H, 3.21; N, 8.99; Cl, 21.15. In the NMR spectrum of the same solid used for elemental analysis dissolved in CDCl3 a signal at 5.302 ppm is clearly visible, confirming the presence of CH2Cl2. MS (FAB+, m/z (relative intensity)): 914 (M+, 100%, 880 ([M - Cl]+, 37%), 746 ([M - ap]+, 24%), 540 ([M - 2ap - Cl]+, 37%). IR (KBr, cm-1): 3057 w, 2924 w, 1596 s, 1540 w, 1478 m, 1460 s, 1428 s, 1336 m, 1289 m, 1254 m, 1210 s, 1160 m, 1073 w, 1017 m, 923 m, 871 s, 762 m, 734 w, 697 s, 648 w, 498 m. Visnear-IR (CH2Cl2, nm): 430, 505, 975. Magnetic Susceptibility Measurements. Magnetic measurements were carried out only on [Ru2(ap)4F][PF6] and [Ru2(ap)4(OH2)][SO3CF3]2, since the counterion in [Ru2(ap)4Cl][FeCl4] is strongly paramagnetic. Method a: Measurement on the Bulk Solid Using a Modified Gouy Technique. In a carefully calibrated tube were placed, in air, 0.023 g of finely ground [Ru2(ap)4F][PF6]. The measurement was carried out several times. From the susceptibility, and after correction for diamagnetic contributions from the ligands, counterion, and displaced air, a magnetic moment, µeff, of 2.75 µB was calculated. Method b: Measurement in Solution by the Evans Method.7 A solution consisting of 0.0119 g of [Ru2(ap)4F][PF6] in 1.3738 g of CH2Cl2 (0.0115 g/mL) was placed, in air, in an NMR tube equipped with an internal reference of neat CH2Cl2. Measurement of the 1H NMR spectrum revealed two broad lines with a separation of 20.2 Hz. For this measurement, and after correcting for diamagnetic contributions, a magnetic moment of 2.90 µB was calculated. In a similar fashion, a solution of Ru2(ap)4(OH2)(SO3CF3)2 in CH2Cl2 with a concentration of 0.0417g/mL was placed, in air, in the insert of an externally referenced NMR tube. The reference, in this case, consisted of a 5% CH2Cl2 solution in CDCl3. The NMR spectrum revealed a sharp line due to unshifted CH2Cl2, and a broad line shifted 48.6 Hz from this. From calculations corrected for diamagnetic contributions a value for the magnetic moment of 2.81 µB was calculated. X-ray Crystallography. A needle-shaped crystal of approximate dimensions 0.1 × 0.05 × 0.02 mm3 was mounted on a quartz fiber by using a dab of hydrocarbon stopcock grease and rapidly transferred to the cold stream of the diffractometer. A detailed description of the procedures employed for this instrument is given elsewhere8 but can be summarized as follows: After optical centering of the crystal in the X-ray beam, 500 reflections were found by automated procedures. The first 50 of these were used to index the crystal, and revealed a triclinic lattice. The structural parameters were then refined from the 250 strongest reflections in the range 10 < θ < 20. Axial rotation images confirmed the observed axial lengths as well as the absence of higher symmetry. Data were collected by integration of 0.2 degree oscillation
images around the ω axis in four different crystal orientations, to measure a total of 12 449 reflections of which 6274 were unique (Rmerge ) 7.4%). The space group P1h was selected since intensity statistics suggested a centrosymmetric space group. The positions of the metal atoms and their first coordination sphere were determined by direct methods as programmed in SHELXS-86,9 and this solution was refined by using the program SHELXL-93.10 All non-hydrogen atoms were found by successive iterations of least-squares refinements followed by Fourier synthesis and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in idealized positions, and a common thermal parameter was refined. Prior to completion of the refinement, the data were corrected for absorption effects using a local modification of the program SORTAV.11 Relevant crystal constants, details of data collection, and structure refinement parameters are given in Table 1. Selected bond lengths and angles are in Table 2, and fractional coordinates for the atoms defining the Ru2N8Cl core in Table 3.
Discussion Synthesis. While the compounds containing the [Ru2(SO4)4(H2O)2]2- anion were prepared by deliberately oxidizing an Ru25+ compound, all the others prepared since have been the result of intended or unintended oxidation by air or the ligand. The current work was prompted by the observation that with appropriate ligands supporting the Ru25+ unit it should be relatively easy to obtain Ru26+ species by simple oxidation of Ru25+ compounds.12 The products obtained from the action of such well known oxidizing agents13 as Ag+ or Cp2Fe+ on the starting Ru25+ compound confirm this. As an additional observation we have found that not only are the diruthenium(5+) tetracarboxylates impervious to such oxidation, but so also are some compounds which are structurally similar to Ru2(ap)4Cl, such as chlorotetrakis(6-chloro-2-hydroxypyridinato)diruthenium(5+). The reasons for this are not known with certainty, but perhaps the basicity of the ligands is so important that the lower basicity of the hydroxypyridinato ligand in comparison with the aminopyridinato ligand makes it unable to stabilize the Ru26+ entity. Structure. A thermal ellipsoid plot of the structure of the cation containing the Ru26+ moiety is shown in Figure 1. Upon (6) The compound crystallyzed in the triclinic space group P1h with a ) 12.811(2) Å, b ) 16.451(1)Å, c ) 12.801(1)Å, R ) 101.44(3)°, β ) 113.61(5)°, and γ ) 79.10(5)°. A total of 11 809 data, of which 6943 are unique (Rint ) 0.0731) and of those 5557 have intensities greater than 2σ(I), were collected. These have been used to refine 617 parameters resulting in residuals of R ) 0.0868, wR2 ) 0.2279 and Goof ) 1.026. The relevant structural parameters are Ru-Ru ) 2.296(1) Å, the average Ru-pyridine-N ) 2.070(5) Å, Ru-amine-N ) 2.009(5) Å, Ru-F ) 2.104(2) Å. (7) Evans, D. F. J. Chem. Soc. 1959, 2003. (8) Bryan, J. C.; Cotton, F. A.; Daniels, L. M.; Haefner, S. C.; Sattelberger, A. P. Inorg. Chem. 1995, 34, 1875. (9) Sheldrick, G. M. In Crystallographic Computing 3; Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University Press: Oxford, England, 1985; pp 175-189. (10) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D., Parkanyi, L., Simon, K. Eds.; Oxford University Press: Oxford, England, 1993; pp 111-122. (11) Blessing, R. H. Acta Crystallogr. 1995, A51, 33. (12) Cotton, F. A.; Ren, T. Inorg. Chem. 1995, 34, 3190. (13) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
Ru26+ Compounds
Inorganic Chemistry, Vol. 36, No. 4, 1997 569
Table 2. Selected Bond Lengths (Å), Angles (deg), and Torsion Angles (deg) for [Ru2(ap)4Cl][FeCl4]‚2.5CH2Cl2 Ru(1)-Ru(2) Ru(1)-N(11) Ru(1)-N(13) Ru(2)-N(21) Ru(2)-N(23)
2.301(1) 2.083(7) 2.089.7 2.012(7) 2.000(7)
Ru(2)-Ru(1)-Cl N(11)-Ru(1)-Ru(2) N(13)-Ru(1)-Ru(2) N(21)-Ru(2)-Ru(1) N(23)-Ru(2)-Ru(1) N(11)-Ru(1)-Cl N(13)-Ru(1)-Cl N(11)-Ru(1)-N(12) N(11)-Ru(1)-N(14) N(12)-Ru(1)-N(14) N(21)-Ru(2)-N(22) N(21)-Ru(2)-N(24) N(22)-Ru(2)-N(24)
Ru(1)-Cl Ru(1)-N(12) Ru(1)-N(14) Ru(2)-N(22) Ru(2)-N(24)
179.84(6) 87.4(2) 87.4(2) 89.0(2) 89.1(2) 92.7(2) 92.4(2) 90.2(3) 89.7(3) 173.8(3) 90.5(3) 89.8(3) 179.2(3)
N(11)-Ru(1)-Ru(2)-N(21) N(13)-Ru(1)-Ru(2)-N(23) av torsion angle
-21.3(3) -21.3(3) -21.3
Table 3. Atomic Coordinates (×10-4) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for the Unique Atoms Defining the Ru2N8Cl Corea Ru(1) Ru(2) Cl N(11) N(21) N(12) N(22) N(13) N(23) N(14) N(24)
x
y
z
U(eq)
3195(1) 818(1) 5697(2) 2454(8) 211(7) 3263(8) 777(8) 3745(7) 1487(7) 2896(7) 886(7)
6924(1) 6915(1) 6930(2) 8026(5) 7660(5) 7628(5) 7926(5) 5817(4) 6173(5) 6222(5) 5895(4)
-2640(1) -2643(1) -2639(1) -2011(4) -1716(4) -3678(4) -3351(4) -3269(4) -3564(4) -1614(4) -1937(4)
19(1) 17(1) 30(1) 21(2) 21(2) 23(2) 24(2) 18(2) 21(2) 21(2) 20(2)
2.419(2) 2.098(7) 2.079(7) 2.002(7) 2.010(7)
N(12)-Ru(1)-Ru(2) N(14)-Ru(1)-Ru(2) N(22)-Ru(2)-Ru(1) N(24)-Ru(2)-Ru(1) N(12)-Ru(1)-Cl N(14)-Ru(1)-Cl N(11)-Ru(1)-N(13) N(12)-Ru(1)-N(13) N(13)-Ru(1)-Nu(14) N(21)-Ru(2)-N(23) N(22)-Ru(2)-N(23) N(23)-Ru(2)-N(24)
87.1(2) 86.7(2) 89.4(2) 90.0(2) 93.0(2) 93.3(2) 174.9(3) 89.6(3) 89.9(3) 178.2(3) 89.6(3) 90.1(3)
N(12)-Ru(1)-Ru(2)-N(22) N(14)-Ru(1)-Ru(2)-N(24)
-21.2(3) -21.3(3)
Table 4. Three Classes of Ru26+ Complexes no. of unpaired electrons 4 0 2 3
complex [Ru2(SO4)4(H2O)2]2Ru2(pfap)4(CCPh)2 Ru2(DPhF)4(CCPh)2 Ru2(DPhF)4(CN)2 Ru2(hpp)4Cl2 [Ru2(ap)4Cl]+ Ru2(ap)4Cl
most probable Ru-Ru (Å) electron confign 2.343 2.441 2.556 2.539 2.321 2.301 2.275
σ2π4δπ*2δ* π4δ2π*4 σ2π4δ2π*2 σ2π4δ2π*2δ*
a U(eq) is defined as one-third of the trace of the orthogonalized U ij tensor.
Figure 1. Thermal ellipsoid drawing of the cation in [Ru2(ap)4Cl][FeCl4]‚2.5CH2Cl2.
oxidation only minor changes in the geometry of the Ru2N8Cl core are observed. The Ru-Ru distance increases to 2.301(1) Å, a 0.026 Å increase from its Ru25+ analogue, while the RuCl distance decreases to 2.419(2) Å, a 0.018 Å decrease. The slight increase is entirely reasonable, since any tendency to decrease the metal-metal distance by the removal of a weakly antibonding electron is compensated by the increased electrostatic repulsion between the metal atoms. This increased electrostatic charge also causes the observed shortening of the
Ru-Cl bond to yield one of the shortest such bonds observed to date.5,12 Since most of the negative charge on the bridging ligands is expected to lie primarily on the amine-N it would be expected that the Ru-amine-N bonds should be shorter than the Ru-pyridine-N bonds, and, indeed, this is so. The Ruamine-N bonds average 2.006(3) Å, and the Ru-pyridine-N bonds average 2.087(4) Å. Both of these are shorter than in the Ru25+ analogue, the difference being 0.017 Å for the Rupyridine-N bonds and 0.020 Å for the Ru-amine-N bonds. These differences are on the order of 3 times the standard uncertainty associated with the measurements. Finally, the N-Ru-Ru-N torsion angles in this molecule, averaging 21.3°, are approximately the same as those in its Ru25+ analogue, with an average of 22.7°. Magnetic Properties. The magnetic properties, as well as the Ru-Ru bond distances, for various Ru26+ complexes fall into three different groups, i.e., those with 4, 2, or 0 unpaired electrons (Table 4). For comparison, the 11-electron analog of [Ru2(ap)4Cl]+ is also included in Table 4. As previously discussed2,3 the sulfato-bridged anion has four unpaired electrons because of the near degeneracy of the δ, π*, and δ* orbitals. The resulting electron configuration, σ2π4δπ*2δ*, means that there is a δ bond order of zero and a π bond order of one. The resulting distance is 2.34 Å. Although, as will be seen, this distance is consistent with the electronic structure for
570 Inorganic Chemistry, Vol. 36, No. 4, 1997 this species as well as with the data for the remaining complexes in Table 4, we can hardly use this complex as a bench mark because it is so different from the others. It also seems unlikely that there will be many (or any) more examples of this unusual electron configuration. The [Ru2(ap)4Cl]+ ion is the first example of an Ru26+ unit bridged by N-C-N- type ligands with only one axial ligand. The recently reported4c Ru2(hpp)4Cl2 is the only other example of an Ru26+ core bridged by N-C-N- type ligands but in this case there are two axial ligands. In these two the Ru-Ru distances, 2.32 and 2.30 Å, are very similar and consistent with the electron configuration σ2π4δ2π*2. This consistency can be established by using the properties of the previously reported Ru2(ap)4Cl (Ru-Ru ) 2.28 Å) as a point of departure. The electron configuration of this molecule is unequivocally σ2π4δ2π*2δ* because it has three unpaired electrons, and no other configuration can account for that. In going from Ru2(ap)4Cl5 to the cation, [Ru2(ap)4Cl]+, we do not change the ligand set at all, we merely remove an electron and raise the charge on the Ru2 core by one unit, from +5 to +6. The increase in positive charge alone would tend to increase the Ru-Ru distance by ca. 0.02-0.03 Å, while the loss of a δ* or a π* electron would cause a contraction of either ca. 0.01-0.02 or ca. 0.08 Å, respectively. Clearly the net result of losing a δ* electron would be a slight (0.01-0.02 Å) increase to ca. 2.29 Å. Conversely, loss of a π* electron would cause a net 0.05-0.06 Å contraction to ca. 2.23 Å. From the actual result, a 0.02-0.03 Å increase, it is clear that a δ* electron is lost. The electron configuration σ2π4δ2π*2 must be correct for the [Ru2(ap)4Cl]+ cation. The same configuration seems correct for the Ru2(hpp)4Cl2 molecule,4c where the slightly greater Ru-
Cotton and Yokochi Ru distance (by 0.02 Å) may be due to the presence of two axial Cl- ligands. This analysis is also consistent with having Ru-Ru distances of ca. 2.42-2.47 Å in Ru2(N-C-N)4 and Ru2(N-N-N)4 molecules1 because of the combined effect of lowering the oxidation state from Ru26+ to Ru24+ and adding two π* electrons to give a σ2π4δ2π*4δ*0 (-0.02 - 0.02 + 0.08 + 0.08 ) +0.12) should take the Ru-Ru distance in an Ru26+ unit, ca. 2.32 Å, to about 2.44 Å in the corresponding Ru24+ unit. Finally, we turn to the three Ru26+ species4a,b that have strongly bound PhCtC- or NtC- axial ligands, no unpaired electrons and very long Ru-Ru distances, 2.44, 2.54, and 2.56 Å. Simply removing two antibonding electrons from a σ2π4δ2π*4 or σ2π4δ2π*2δ*2 configuration could not possibly lengthen the Ru-Ru distance from ca. 2.30 Å to something in the range of 2.44-2.56 Å. So, how can we account for the experimental observations? We can do this only by removing the Ru-Ru σ bond, which frees the dz2 orbitals to form very strong Ru-C bonds and gives a π4δ2π*4 configuration. Since this has only a net δ bond, the Ru-Ru distance will be very long. Acknowledgment. We thank the NSF for support, Dr. C. A. Murillo for helpful discussions, and Mr. J. H. Matonic and Dr. G. T. Jordan for assistance with NMR spectroscopy. Supporting Information Available: Tables of fractional atomic coordinates, bond lengths and angles, anisotropic displacement parameters, and hydrogen atom coordinates are avaliable as an X-ray crystallographic file in CIF format. Access information is avaliable on any current masthead page. IC960637C
